Process for making semisolid cryogens



6, 1968 H. B. SHERLOCK ET AL 3,395,546

PROCESS FOR MAKING SEMISOLID CRYOGENS Filed July 31, 1964 2 Sheets-Sheet1 TfMPERA 7UR E/VTROPY (5) FIG. 4.

T E MPERA TUBE 090/0 -V/IPOR E/WAOP) (S) INVENTOR5 H. BENTLEY SHERLOCKNEWMAN E. STANLEY g- 5, 1968 H. B. SHERLOCK ET AL 3,395,546

PROCESS FOR MAKING SEMISOLID CRYOGENS 2 Sheets-Sheet 2 Filed July 31,1964 m. 4 1 v V Ha l l 19 TTORNEKS United States Patent 3,395,546PROCESS FOR MAKING SEMISOLID CRYOGENS Harrison Bentley Sherlock andNewman E. Stanley, St. Louis County, Mo., assignors to McDonnellAircraft Corporation, St. Louis County, Mo., a corporation of MarylandFiled July 31, 1964, Ser. No. 386,618 11 Claims. (Cl. 62-10) The presentinvention relates to the process of making semi-solid refrigerants andmore particularly to a process for preparing semi-solid cryogens whichhave become commonly known as slush cryogens.

In recent times both advanced aircraft and spacecraft are usingcryogenics in more and more applications. Typically, cryogenics arebeing used for breathing oxygen, prime moving energy source, pressurantgas, fuel for fuel cells, and as energy sources for secondary powersystems. The most crucial problems associated with the handling ofcryogenics concerns the loading of high quality, single phase liquid onboard the craft during preflight servicing. At best, due to heat flux,friction pressure drop and various other heat imparting operations tothe cryogenics, such operations are sufiicient to initiate two phaseconditions and consequent degradation of the cryogenics liquid quality.Hence, instead of a pure liquid phase, much gas entrainment occurswithin the liquid causing reduced quantities of cryogenic liquid onboard the craft after servicing. Moreover, the problem of cryogenicliquids developing more of a gas phase continues after the initialservicing cool down and filling operation because of heat losses throughthe vehicle storage tanks, and likewise such increasing gas phaseprevents retention of good quality liquid cryogens on board the craft.

Furthermore, some cryogenics, having low density and low boiling point,are undesired because of the current difficulty in providing such liquidcryogen on board the craft. Hydrogen is typical low density and boilingpoint cryogen.

Heretofore the problems of cryogenic liquids having a gas phase whenloaded on board the aircraft or spacecraft were avoided by sub-coolingthe cryogen. Sub-cooling was accomplished through heat exchangessubmerged in a different cryogen whose normal boiling point issubstantially lower than the cryogen being cooled. Autogenous cooling bylow pressure boil off that pulls a vacuum over the liquid is anotherpopular method of sub cooling cryogens. Still a third technique insub-cooling cryogens is the lowering of the vapor pressure by injectionof a non-condensible gas. These methods are not without difliculties.For example, inexpensive cryogenic heat sinks are not available and anairborne Dewar flask full of saturated cryogen would overflow under gasinjection.

The present invention avoids the difficulties involved in providingsuitable cryogens on board aircraft and spacecraft by providing cryogensas a semi-solid or slush cryogen. The process appertaining to theinvention herein modifies the Brayton cycle to an open cycle which canbe adapted to batch or continuous operation for producing slushcryogens. Briefly, the process of the invention advantageously employsconstant entropy compression of the cryogen to a higher pressure,cooling the high pressure of cryogen by transferral of the cryogen heatcontent into a heat sink of the same cryogen maintained at the normalboiling point, allowing the cryogen at high pressure to solidify atleast partly, and thereafter expanding the frozen cryogen at constantentropy to the initial pressure. Typically, the modified open Braytoncycle for hydrogen operates between normal atmospheric pressure and 5000p.s.i.a.

It will be understood that by providing a high pres- 3,395,546 PatentedAug. 6, 1968 sure pump the liquid cryogen can be compressedisentropically continuously at one end of a transfer line andcontinuously transferred at the high pressure through the line Whileundergoing solidification. As the distance transferred increases, theamount of freezing increases, then the frozen cryogen is passedcontinuously through an expansion engine having a high pressure inletand being discharged as slush cryogen into appropriate containers inservicing areas for aircraft or spacecraft. It will be appreciated forthe continuous process and transfer system appertaining to the inventionthat the further distance the cryogen is transferred the more freezingoccurs. Such increased cooling with increase trans-fer distance is notpossible in usual transfer systems.

It is therefore an object of the invention to provide a process formaking semi-solid cryogen which is simple and easy to perform and may beoperated as a batch process or a continuous process.

It is another object of the invention to provide a modified open Braytoncycle for the manufacture of slush cryogen.

-It is another object of the invention to provide a method of makingsemi-solid refrigerants by a refrigeration cycle utilizing constantentropy compression, cooling of the compressed refrigerant and constantentropy expansion of the refrigerant, such cycle forming semi-solidrefrigerant.

It is another object of the invention to provide a method of makingsemi-solid refrigerant utilizing a constant entropy compression andexpansion process in which the refrigerant is transferred through a heatexchanger of the same refrigerant material maintained at its normalboiling point intermediate between the constant entropy compressionmachine and constant energy expansion machine.

Still another object of the invention is to provide a method of makingslush hydrogen from saturated liquid hydrogen at atmospheric pressure bysubjecting the liquid hydrogen to constant entropy compression to a highpressure, cooling the pressurized liquid hydrogen in a bath of liquidhydrogen maintained at the normal boiling point to freeze the highpressure liquid hydrogen, and thereafter expanding at constant entropythe solid high pressure hydrogen generating semi-solid hydrogen.

Still another object of the invention is to provide a continuous processfor preparing and transferring semisolid hydrogen by compressing liquidhydrogen at constant entropy to a high pressure and forcing the liquidthrough a transfer line and passing liquid hydrogen at its boiling pointin an annular passage of the transfer line, the high pressure hydrogenfreezing by the heat absorption of the liquid hydrogen vaporizing,discharging the high pressure frozen hydrogen through an expansionengine to reduce the hydrogen to semi-solid state at atmosphericpressure.

A further object of the invention is to provide a method for generatingliquid hydrogen containing about 68 percent soild hydrogen comprisingcompressing isentropically saturated liquid hydrogen at its boilingpoint and atmospheric pressure, to about 5000 p.s.i.a. and exposing thehigh pressure liquid hydrogen to a boiling hydrogen bath until the highpressure liquid hydrogen freezes throughout and then isentropicallyexpanding the solid hydrogen to atmospheric pressure.

These and other objects and advantages of the invention will becomeapparent from the detailed description and the appended claims inconjunction with the drawings wherein:

FIG. 1 is a temperature-entropy diagram of the open Brayton cycleappertaining to the invention;

FIG. 2 is a schematic diagram of batch process for making semi-solidrefrigerant;

FIG. 3 is a schematic diagram of apparatus for the continuous process ofmaking semi-solid refrigerant; and

FIG. 4 is a temperature-entropy diagram depicting the process forcontinuous preparation and transfer of semisolid refrigerant.

Referring specifically to FIG. 1, the modified open Brayton cycleappertaining to the invention will be described. The curve L has asaturated liquid line for the cryogen under study. The condition ofliquid to the right of line -L is part liquid and part vapor. Curve F isthe fusion line for liquid cryogen at which entropy it begins to turn tosolid. Curve M is the solidus line for the refrigerant. Between curves Mand F both solid and liquid refrigerant occurs. Thus by producing amaterial with the entropy between lines M and F we have semi-solid orslush refrigerant. Assuming a given liquid at point A is at atmosphericpressure and has an entropy S then after being compressedisentropically, the liquid will have the same entropy S but will be at atemperature of point B. Next, the compressed liquid at point B is cooledthus decreasing in entropy, and at point C where it has cooled to thefusion temperature, the liquid has a new entropy S When maintained atthe fusion temperature for a varying period of time the entropy of theliquid changes from S and approaches the solidus line at point D with anentropy of S Point D represents fusion of all of the high pressureliquid refrigerant to a solid. Any further decrease in entropy from Swill be accomplished by a decrease in temperature of the solid. At pointD solid refrigerant exists at a high pressure and the fusion temperaturewith an entropy S On constant entropy or isentropic expansion of thesolid at point D to atmospheric pressure, the refrigerant material willbe at point E with an entropy S which is identical with the entropy SHowever, refrigerant at point B and atmospheric pressure has an entropyhigher than that of the solid refrigerant represented by atmosphericpressure. Therefore, after isentiropically expanding the refrigerantfrom its solidus temperature at point D, part of the solid has meltedand consequently point B represents the semi-solid refrigerant or slushrefrigerant.

Referring now to FIG. 2 a schematic representation of the batch processwill be described. The batch system 10 includes a central cylinder 1surrounded by an outer cylinder 2. At one end of cylinder or pipe 1 is apiston 3 and at the same end nipple 4 is attached to pipe 1. The nipple4 has a three Way valve V which couples nipple 4 to a source of highpressure helium, or alternatively, to a vent. The opposite end of pipe 1has a valve V closing tube 1. Outer cylinder 2 has a liquid hydrogeninlet 5 and a hydrogen vapor outlet 6. The entire surface of outercylinder 2 is surrounded by insulation 7 to reduce heat transfer betweenthe outer cylinder 2 and the surrounding environment to a minimum. Aliquid hydrogen inlet, line 8, including valve V controls the liquidhydrogen flow into inner pipe 1.

Referring to FIGS. 1 and 2, the operation of the batch process will nowbe described. Utilizing hydrogen as a refrigerant or cryogen and theconditions set forth as points A, B, C, D and E, the manufacture ofslush hydrogen will be noted. Saturated liquid hydrogen at atmosphericpressure and 20.4 K., represented by point A in FIG. 1 is supplied. Withvalve 1 closed, valve 2 open and valve 3 closed, the liquid hydrogen isthen introduced into inner cylinder 1, conditions represented by pointA. Next valve V after filling inner cylinder 1 and outer cylinder 2, isclosed. Valve V is open to apply pressure against piston 3. With heliumat 5000 p.s.i., piston 3 compresses the hydrogen in pipe 1 against valveV Thus the liquid hydrogen is at the condition represented by point B asseen in FIG. 1. Next, the liquid hydrogen in outer cylinder 2 beginsabsorbing heat from the compressed hydrogen in inner cylinder 1 at pointB. After the work of compression is rejected into the liquid hydrogen inouter cylinder 2 the temperature of the liquid hydrogen in cylinder 1 isreduced at constant pressure to the fusion temperature represented atpoint C of FIG. 1. As the hydogen compressed in pipe 1 further rejectsheat to the boiling liquid hydrogen in outer cylinder 2, the liquid incylinder 1 approaches the solidus point D in FIG. 1. Thus, it will benoted that from point B to point D the hydrogen has decreased in entropyfrom S to a value S After isentropic expansion by opening valve V tovent, open valve V to discharge the slush hydrogen in inner cylinder 1,the slush hydrogen is at point B of FIG. 1.

Referring now to FIG. 3, a schematic representation of the continuousprocess apparatus 20 for generating and transferring slush cryogen willbe described. In the description hydrogen will be used to indicate asuitable cryogen. The system includes a liquid hydrogen source 21connected by a valve 22 to a high pressure pump designated generally bythe numeral 30. Liquid line 23 connnects valve 22 into a by-pass line 24having an intermediate valve 25 into the outer jacket 26 of transferline 40. The entire line 40 and other portions of the system aresurrounded by insulation 43. Line 23 opens into a high pressure pump 30which is depicted symbolically and may be a rotary pump or othersuitable type with high pressure discharged into the inner channel 41 oftransfer line 40 at pressure up to 5000 p.s.i.g. Outer jacket 26 has ahydrogen vapor discharge line 42 near expansion engine which coupleschannel 41 through rotary expansion wheel 51 to atmospheric dischargeline 52. The engine 50 is a roller supported vane expander.

Referring now to FIG. 4 the entropy diagram of the continuous processschematically represented in FIG. 3 is illustrated. It will be notedthat the entropy diagram of FIG. 4 is substantially identical to theentropy diagram of FIG. 1 with the exception that the entropy changefrom the saturated liquid fusion line at point C to the saturated solidline at point D has intermediate points D D D and D D represents a pointat which transfer line 40 is relatively short and consequently thesaturated liquid of point C decreases in entropy to D; at which pointthe liquid including solid has reached the expansion engine 50 andundergoes constant entropy expansion from point D; to point E Similarlyin the long system the saturated liquid at point C will decrease inentropy and freeze at point D. Thus, it will be observed that the longsystem is preferably a length at which the entropy just reaches point Dbefore transfer through the expansion engine at constant entropy topoint B. Intermediate points D and D D represent intermediate lengthsfor the system between a short system and a long system and in each caseconstant entropy expansion to points E E and E respectively, would beaccomplished in the expansion engine 50.

From the above description of the continuous process and transfer ofslush cryogens it will be noted that the entropy of the cryogendischarged from the expansion engine will be much lower for the longsystem as compared to the short system. This is a unique advantageafforded by the invention hereof because the system avoids the problemof cryogens for aircraft and spacecraft gaining entropy during transferfrom the place of manufacture or storage to the servicing area, loadingarea, or using area.

It will be appreciated that various embodiments have been utilized todescribe the invention and many changes and modifications will bereadily apparent and will occur to those skilled in the art which do notdepart from the spirit and scope of the invention; and such changes asthese are deemed to be within the scope of the present invention whichis limited solely by the scope of the appended claims.

What is claimed is:

1. A method of making semi-solid refrigerant without a vacuum comprisingthe steps of isentropically compressing a liquid refrigerant from aninitial to a higher pressure, both pressures within the range fromatmospheric to 5000 p.s.i., cooling the compressed liquid refrigerant atsubstantially constant pressure to a point having an entropy less thenthe triple point entropy by transfer of the heat of compression to aboiling liquid body of the refrigerant, allowing the compressed andcooled liquid refrigerant to freeze, and isentropically expanding thefrozen refrigerant to the initial pressure thereby permitting part ofthe frozen refrigerant to melt.

2. A method of making semi-solid cryogen Without a vacuum comprising thesteps of isentropically compressing liquid cryogen to a higher pressure,cooling the compressed liquid cryogen at substantially constant pressureto a point having an enropy less then the triple point entropy bytransfer of heat therefrom to a vaporizing body of the cryogen,continuing heat removal from the compressed cryogen until partlysolidified, and isentropically expanding the partly solidified cryogenthereby creating semi-solid cryogen.

3. The method of making semi-solid cryogen in claim 2, wherein thecryogen is selected from the group consisting of hydrogen, nitrogen,fluorine, argon and neon.

4. The method of making a semi-solid cryogen comprising the steps offorming a saturated liquid body of cryogen at atmospheric pressure,compressing the cryogen to a substantially higher pressure, cooling thecryogen at substantially constant pressure to the fusion temperature byheat transfer exposure to a boiling liquid body of the same cryogen,removing heat of fusion from said liquid cryogen at substantially thesame pressure to a point having an entropy less than the triple pointentropy, and insentropically expanding the solid cryogen to atmosphericpressure thereby melting part of the solidified material.

5. The method of making semi-solid hydrogen comprising the steps ofcompressing hydrogen liquid isentropically from atmospheric pressure,rejecting the heat of compression at substantially constant pressureinto a body of liquid hydrogen maintained at 37 R., decreasing theentropy of the hydrogen liquid at substantially the same pressure to apoint having an entropy less than the triple point entropy to causesolidification thereof, and releasing the pressure on the solidifiedhydrogen liquid to allow isentropic expansion into atmospheric pressureto form a semi-solid hydrogen.

6. The method of continuously making semi-solid refrigerant comprisingthe steps of continuosly isentropically compressing a flowing liquidcryogen to a high pressure, passing the continuous flowing liquidrefrigerant at substantially constant pressure through a heat removalpath provided by a vaporizing liquid body of the refrigerant for aperiod of time sufficient to solidify at least some of the refrigerantat a point having an entropy less than the triple point entropy, passingthe high pressure at least partly solidified refrigerant through anexpansion machine which isentropically expands the refrigerant,discharging at least partly solidified refrigerant.

7. Method of continuously making and transferring semi-solid cryogencomprising the steps of continuously introducing liquid cryogen into anisentropical compressor stage, discharging the liquid cryogen from thecompressor at a high pressure into a transfer line, said transfer linesurrounded by a body of the liquid cryogen at its vaporization point,flowing the high pressure cryogen at substantially constant pressurethrough said transfer line for a distance sutficient to cause at leastsome solidification of the high pressure liquid cryogen at a pointhaving an entropy less than the triple point entropy, and introducingthe semi-solid cryogen into an isentropical expansion engine thatdischarges the semi-solid cryogen at a pressure equal to the inletpressure of the isentropical compression stage.

8. The method of continuously making semi-solid cryogen comprising thesteps of continuously compressing a stream of liquid cryogen at aconstant entropy, super cooling the cryogen at substantially constantpressure to an entropy state below the entropy of the atmosphericpressure fusion point and below the triple point entropy, andcontinuously isentropically expanding the liquid cryogen containingsolid cryogen to atmospheric pressure to form a semi-solid cryogen whoseentropy is less than the entropy at the fusion point without a vacuum.

9. A method of making semi-solid cryogen without using vacuum comprisingisentropically compressing a liquid cryogen from atmospheric pressure toa higher pressure and increasing the temperature of the liquid; coolingthe liquid at said higher pressure and reducing the entropy until thefusion line is reached; freezing the liquid at said pressure until atleast a portion of solids are formed and the entropy of the fluid isbelow the triple point; isentropically expanding the material toatmospheric pressure to melt some of the solids, and recovering asemi-solid slush at the said entropy.

10. The process of claim 9 wherein the cryogen is hydrogen.

11. The process of claim 9 wherein the liquid is frozen substantiallycompletely solid before being expanded isentropically.

References Cited UNITED STATES PATENTS 3/1966 Null et a1 6210 X OTHERREFERENCES Hydrogen Subcooling for Aerospace Vehicles, Elrod,

IEEE Transactions on Aerospace, vol. AS-l, No. 2,

NORMAN YUDKOFF, Primary Examiner. W. PRETKA, Assistant Examiner.

1. A METHOD OF MAKING SEMI-SOLID REFRIGERANT WITHOUT A VACUUM COMPRISINGTHE STEPS OF ISENTROPICALLY COMPRESSING A LIQUID REFRIGERANT FROM ANINITIAL TO A HIGHER PRESSURE, BOTH PRESSURES WITHIN THE RANGE FROMATMOSPHERIC TO 5000 P.S.I., COOLING THE COMPRESSED LIQUID REFRIGERANT ATSUBSTANTIALLY CONSTANT PRESSURE TO A POINT HAVING AN ENTROPY LESS THENTHE TRIPLE POINT ENTROPY BY TRANSFER OF THE HEAT OF COMPRESSION TO ABOILING LIQUID BODY OF THE REFRIGERANT, ALLOWING THE COMPRESSED ANDCOOLED LIQUID REFRIGERANT TO FREEZE, AND ISENTROPICALLY EXPANDING THEFROZEN REFRIGERANT TO THE INITIAL PRESSURE THEREBY PERMITTING PART OFTHE FROZEN REFRIGERANT TO MELT.